Studio chat: minimap, side panels, and session inspection #1850

Parent: #1736. Design review: #1779.
Design baseline: 2026-09-24.

Outcome

Users can navigate a long conversation and inspect its related session, file, and workspace details without losing the active chat.

Scope

• Add a message minimap, per-message reply thread, file and tool previews, local Markdown canvas, and a consistent resizable side-panel shell.
• Add session details, transcript, soul, worktree picker, and debug dialogs where the runtime advertises the capability. Add read-only access to inspect-only sessions without presenting them as editable chats.
• Introduce narrow BFF and contract routes for soul and session worktrees as needed. Soul is connection-scoped; worktrees are session-scoped. Keep paths and credentials out of public placement inference.

Acceptance

• A minimap target scrolls its own message into view even near the start of a long transcript; panel and dialog state survives ordinary transcript updates.
• Capability-disabled inspection affordances explain their state, and all new routes remain authenticated and follow the existing generated-contract pipeline.
• Focused BFF/UI tests, desktop/mobile screenshots, and task studio:check pass.

Delivery

Bounded contract/UI plan; any placement or trust-boundary change requires its own architecture decision.

Depends on: #1847, #1849.